In 3GPP there is an initiative ongoing to define next generation architecture for PS based services. This work goes under the work item, SAE (System Architecture Evolution). In this context a new architecture for mobile networks is discussed. The work is focused on a simplified and more cost effective architecture, such that fewer nodes are needed in the network. The focus is also on more effective protocols and support for services that have higher performance requirements. Less delays and higher throughput are important issues. Excessive delays are present when traffic needs to be routed ineffectively e.g. via a Home network while the user is roaming in a visited PLMN (VPLMN).
The routing via Home network is called tromboning effect and is well known from the CS (circuit switched) domain, where telephone calls may be routed from a VPLMN, through the HPLMN (Home PLMN) to the B party end user. There may e.g. be cases for VoIP (Voice over IP) and IMS (IP multimedia subsystems) where a call is made to a peer user who is present in the same VPLMN as the calling user, but the traffic has to be sent via IMS and the GGSN in the HPLMN. It is not optimal.
Roaming users are billed by the Home operator. When the GPRS user is roaming, the SGSN in the VPLMN will perform traffic measurements and counting the amount of traffic that is handled. The traffic reports are sent to the Home Operator as CDR's (Charging Data Records) for accounting between the operators. In the current solution the traffic also passes the HPLMN, where the Home Operator generates his own CDR's, and there is a double check that the traffic charges that the Visited Operator claims from the Home Operator (and also the billing of the user) are correct.
The currently existing 3GPP specified GPRS architecture is well designed for PS (Packet Switched) applications with moderate requirements for latency, but not so for delay sensitive real time applications such as VoIP or gaming. For these types of applications the Home routing (tromboning) may be a problem.
A solution to the routing problem and the trombone effect would be the so called local breakout method (or optimal routing in the CS domain), instead of routing the PS traffic from the VPLMN, through the HPLMN and the Home GGSN and out to Internet or to an other network, the traffic is sent directly to the external network or the service network from the VPLMN, see route A in FIG. 1.
However, the local breakout method may be attractive from a traffic plane consideration, but from the control plane point of view there are less attractive effects. A new problem appears. When the traffic is routed directly out from the VPLMN, there are no control plane activities possible in the HPLMN. The Home Operator will not be able to do traffic metering or any control of the traffic flow. However, the Home Operator will still have to bill the user, but in this case based entirely on CDR's generated by the Visited Operator. The Home Operator will have no means of checking upon the claims of the Visited Operator. These circumstances call for a strong trust relationship between the Home and the Visited Operators.
Without solving the billing trust problem, the local breakout method may not be allowed by the operators.